1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor element, by which an element isolation layer is formed by LOCOS (Local Oxidation of Silicon) in a semiconductor layer of an SOI (Silicon On Insulator) substrate or an SOS (Silicon On Sapphire) substrate.
2. Description of the Related Art
As is well known in the art, a technique for fabricating a semiconductor device using the SOI substrate or the SOS substrate can facilitate complete isolation between semiconductor elements formed in the semiconductor layer, thereby preventing soft errors or latch-ups. The technique can also reduce the junction capacitance of the source-drain region. Accordingly, the technique serves to realize higher operation speeds and reduced power consumption, and is thus often employed to manufacture semiconductor devices.
In recent years, as increasingly higher density semiconductor devices are demanded, finer design rules are applied to manufacture semiconductor elements mainly using the SOI substrate or the SOS substrate that includes a semiconductor layer of a thickness of 50 nm (nanometer) or less in which a semiconductor element is formed.
On the other hand, the LOCOS technique is available as one of the methods of forming an element isolation layer on a typical silicon substrate to isolate dielectrically between semiconductor elements. This method is also employed to form an element isolation layer in a relatively thick-film silicon layer.
The element isolation layer can be typically formed in such a relatively thick-film silicon layer as follows. That is, before the element isolation layer is formed, a silicon nitride film is formed on the silicon layer via a pad oxide film. These films are then etched by anisotropic etching to expose the silicon layer, and the exposed silicon is oxidized by LOCOS to form an insulating film of silicon dioxide (SiO2).
On the other hand, a thin-film semiconductor layer is etched as follows. That is, to form a sidewall spacer of silicon dioxide on the sidewall of the gate electrode of a MOS element, a 70 to 90% of the thickness of the silicon dioxide film formed on top of the gate electrode is etched. Thereafter, the remaining silicon dioxide film is removed by plasma etching with a high selection ratio against silicon, thereby removing the silicon dioxide film on top of the gate electrode. Thus, formation of tails is prevented which would be otherwise caused by deposition of polymerized film on the sidewall portion of the gate electrode (e.g., Japanese Patent Laid-Open Publication No. 2002-237603 (Page 5, Paragraph 0031 to Paragraph 0036, and FIG. 6.)
Furthermore, the applicant has suggested a technique in Japanese Patent Application No. 2004-332695 as a method of forming an element isolation layer in a thin-film semiconductor layer. In this method, a silicon nitride film that has been formed on a semiconductor layer via a pad oxide film is etched, but not completely, in the first etching process with a high etching rate for the silicon nitride film. Thereafter, in the second etching process with a high selection ratio against silicon, the semiconductor layer is exposed, and then the element isolation layer is formed in the exposed semiconductor layer by LOCOS.
The aforementioned typical method of fabricating the element isolation layer causes degradation in reliability of the semiconductor device because of the following reasons. That is, the semiconductor layer reduced in thickness down to 50 nm or less due to finer design rules for the semiconductor device would increase the risk of removing the semiconductor layer when part of the silicon nitride film (an oxidation-resistant mask layer) that has been deposited before the element isolation layer is formed is etched by anisotropic etching. Thus, when the semiconductor layer is not left but removed, it is difficult to form an insulating film of silicon dioxide by LOCOS, thereby causing incomplete dielectric isolation between semiconductor elements and thus leading to short circuits between the adjacent semiconductor elements.
On the other hand, even when the semiconductor layer is left, the remaining semiconductor layer is thinner than the semiconductor layer below the silicon nitride film that has been left as a mask. This may cause a side of the semiconductor layer below the silicon nitride film left as the mask to be exposed, and thus the side would be directly oxidized. As a result, a bird's beak would be readily formed, resulting in an effective transistor formation region being reduced.